Rearranging a 4d numpy array

Question

I have a 4d numpy array which represents a dataset with 3d instances. Lets say that the shape of the array is (32, 32, 3, 73257).

How can i change the

Answer 1

It looks like np.rollaxis(arr, axis=-1) will do what you want. Example:

>>> arr = np.empty(32, 32, 3, 73257)
>>> arr2 = np.rollaxis(arr, axis=-1)
>>> arr2.shape
(73257, 32, 32, 3)

This will make arr[i,j,k,l] == arr2[l,i,j,k] for all ijkl

Answer 2

The np.transpose function does exactly what you want, you can pass an axis argument which controls which axis you want to swap:

a = np.empty((32, 32, 3, 73257))
b = np.transpose(a, (3, 0, 1, 2))

The axis of b are permuted versions of the ones of a: the axis 0 of b is the 3-rd axis of a, the axis 1 of b is the 0-th axis of a, etc...

That way, you can specify which of the axis of size 32 you want in second or in third place:

b = np.transpose(a, (3, 1, 0, 2))

Also gives an array of the desired shape, but is different from the previous one.